A UPS (Uninterruptible Power Supply) is an alternating current power supply with an energy storage device, which provides uninterruptible power to a load using the energy storage device such as a battery during power failures. When the mains power supply is available, the UPS regulates the mains power supply and supplies it to the load. When the mains power supply is not available (power failure), the UPS supplies alternating currents (AC) to the load using the energy storage device, so as to keep the load working normally and protect the software and hardware of the load from damage.
In order to filter the power input, the UPS commonly adopts a structure for AC/DC/AC conversion to provide a pure electric power to the load. Wherein, the AC/DC link converts the AC mains power supply into a smooth DC bus voltage and then provides energy to the next DC/AC link. A conventional AC/DC is realized by uncontrolled rectifier diodes or half controlled SCR and has a low input power factor and a high current harmonics, which will pollute the power grid. A modern AC/DC adopts active semiconductor devices, which can smooth the DC bus voltage and correct the power factor, and thus reduce the pollution to the power grid and ensure a green power grid. A DC/AC link is used to convert the DC bus voltage originating from the AC/DC or the electricity energy storage device into a pure sinusoidal output voltage, so as to ensure the power supply quality and uninterrupted electricity for the load.
FIG. 1 shows a single phase 2 wire UPS circuit with a single phase line L and a single neutral line N, from which the AC/DC/AC conversion process of this UPS circuit is clear. As shown in FIG. 1, the single phase 2 wire UPS circuit comprises a rectification phase leg PL1 for rectification and an inversion phase leg PL2 for inversion. Gate drivers G control the rectification phase leg PL1 and inversion phase leg PL2 corresponding thereto in response to a PWM signal. The AC mains power supply is transmitted from an input end Tin to the load of the UPS through a rectification inductor Lrec, the rectification phase leg PL1, the inversion phase leg PL2, an inversion inductor Linv and an output end Tout.
FIG. 2 shows a 3 phase 3 wire UPS circuit with three phase lines L1, L2 and L3, which comprises three rectification phase legs PL1 and three inversion phase legs PL2 controlled by the corresponding gate drivers respectively. FIG. 3 shows a 3 phase 4 wire UPS circuit with three phase lines L1, L2, L3 and one neutral line N, the structure of which is similar to that of the 3 phase 3 wire UPS circuit shown in FIG. 2, except for having a further neutral line N.
Reliability and efficiency are two of the most important performances of the UPS. At present, in order to obtain higher efficiency, the inversion phase legs are generally configured into a multi-level conversion topology which requires many semiconductor devices. However, the more the semiconductor devices are, the poorer the reliability is. And thus the reliability of the inversion phase legs decreases. Since the inversion phase legs are common modules of the mains power supply mode and the battery mode, the decrease of the reliability will severely affect the reliability of the whole UPS.